The present invention relates to an apparatus for developing latent electrostatic images.
The current most used system for developing a latent electrostatic image for so-called "office copier" machines involves the steps of uniformly charging a photoconductor surface, exposing it to an imagewise radiation and developing the exposed surface by depositing colored particles on the latent images. Shortcomings of this dry developer are that the colored particles cause dust that stains delicate parts of the copying machine and that the deposited particles must be fixed by applying heat and pressure. Another system that has been proposed to overcome these shortcomings is one that employs an organic developing liquid of high electrical resistivity in which colored particles are dispersed. A recording medium carrying latent electrostatic images is submerged into the liquid. Development of liquid occurs when the colored particles are deposited on the latent images through what is known as "electrophoretic" process which is followed by drying the organic liquid. One shortcoming of this type of development is the deposition of colored particles in the background where no such deposits should be present. In addition, the drying process tends to warm the organic liquid, causing it to evaporate. To solve these problems, U.S. Pat. No. 4,202,620 discloses a liquid development apparatus in which a film of developing liquid is formed on the surface of a roller submerged in the liquid and moved past a latent electrostatic image that is formed on a photoconductor surface. As the film is brought close to the latent image, electrostatic fields develop and the liquid bulges by attraction from its surface and adheres to the charged image. Since the disclosed apparatus allows the use of water-soluble developing liquid, the drying process does not result in the vaporization of organic solvent.
However, one shortcoming of the prior art resides in the fact that field concentrations can occur in localized areas of a relatively wide, high density (colored) area, resulting in nonuniform distribution of field intensities, and hence "white blots" in an otherwise uniformly colored area. Furthermore, a localized field concentration accelerates its process and results in the clustering of small bulges into a single lump, and hence loss of fine details.